The present invention relates to non-contact systems for detecting the level of a liquid metal bath; although it may be used in any system where it is desired to measure the distance between an inductive coil and a metal object, whether that object be ferrous or non-ferrous. The invention finds specific application in detecting the level of a molten aluminum bath in continuous casting molds.
An inductive non-contact distance measurement system for liquid metal bath levels is desired, for example, in Yamada et al U.S. Pat. No. 4,030,027, issued Jun. 14, 1977. In that patent, an inductive coil is employed in the feedback circuit of an operational amplifier having two input terminals, one of which is connected to an oscillator and the other of which is connected to the aforesaid feedback circuit. The amplified differential output signal from the amplifier, comprising the difference between the oscillatory signal applied to the differential amplifier and the signal applied to the differential amplifier which varies as a function of the impedance of the detecting coil corresponding to the distance between the detecting coil and the metallic body, are a measure of the distance between the two. At least one of the parameters of open-loop gain of the differential amplifier and the amount of feedback in the feedback circuit are predetermined so as to linearize at least for a predetermined measuring range the output characteristics of the differential amplifier resulting from the variation of the detecting coil impedance caused by the variation of the distance.
Other systems of this type have attemped to provide a linearizing change in the detector circuit for changes in the permeance of the detector coil with the use of bridges, phase-angle correction circuits, and the like. These earlier circuits incorporated the linearizing correction upstream of the first operational amplifier or in the feedback path, exemplified by the aforesaid U.S. Pat. No. 4,030,027.